Donkeys experimentally infected with Trypanosoma brucei showed dullness, weakness, fever, inappetence, conjunctivitis, tachycardia and polydyspnoea soon after detectable parasitaemia. The parasitaemia was generally low with transient high peaks except in the terminal stage when there was sustained high parasitaemia. A moderate anaemia was present as from the second week of infection but it was not progressive. There was a marked leucopoenia within 24 h of patent parasitaemia. Death occurred 2 to 2 1/2 months after infection and at necropsy there was severe emaciation as well as mild serous effusion. Histologically, there was a nonsuppurative encephalomyelitis, cranial neuritis, extensive haemosiderosis, hyperplasia of follicles in lymph nodes and spleen and giant cell reaction in lymph nodes. Trypanosomes were present in the cerebrospinal fluid, the eye and serous effusions. These observations are similar to those previously reported in other animals infected with T. brucei.